Curved mirrors commonly are employed as rearview mirrors for motor vehicles, as reflecting surfaces for telescopes, and the like. Curved mirrors commonly are formed by first forming a glass sheet or other sheet-like substrate into the desired curved configuration, and subsequently applying a reflective coating to one side or the other of the substrate. For example, curved mirrors of the type used in carnivals to provide amusing, contorted reflections of a viewer may be made by first forming a sheet of glass into the desired shape, and then coating one surface of the glass with metallic silver and a protective paint overcoat.
Mirrors also can be manufactured by employing a magnetron sputtering technique such as that described in Chapin, U.S. Pat. No. 4,166,018. Mirrors of this type may use chromium or silver as the reflective layer. When curved mirrors are manufactured using a magnetron sputtering process, the glass substrates for the mirrors are first bent as desired typically in a size that would produce two or more mirrors. After the bent glass pieces are washed in a curved glass batch-type washer or on a carrier in an on-line washing system, they are placed on an appropriate carrier and are coated by magnetron sputtering. Due to the curvature of the or aluminum, niobium being preferred. The third layer is a protective film that is positioned further from the substrate than the reflective layer, the protective film providing sufficient oxygen permeation inhibition as to prevent the reflectance of the mirror from decreasing to less than 50% upon heat bending. The third layer preferably comprises sputter-deposited silicon nitride, sputter-deposited aluminum oxide or sputter-deposited silicon dioxide; of these, silicon nitride is preferred.
When a heat-formable mirror of the invention is heat formed at a temperature above the temperature at which the layers of the reflective coating are deposited, atomic diffusion and/or structural rearrangements can occur between the various sputtered films, changing the reflective properties of the bent mirror product. The heat formable mirrors of the invention, however, largely and preferably fully retain their important mirror optical properties (low transmissivity, high reflectance) when subjected to heating and bending in this manner.
Thus, in another embodiment, the invention relates to a curved mirror that is produced by providing a heat-formable mirror of the type described above, and subjecting the mirror to a temperature at which the substrate is capable of plastic deformation (e.g., the glass transition temperature in the case of glass substrates), bending the flat mirror at that temperature into a desired curved conformation to produce a curved mirror, and then cooling the curved mirror while maintaining its curved conformation. The resulting curved mirror desirably retains at least about 100% of the reflectance and not over about 150% of the transmissivity of the heat-formable flat mirror from which it was made.
Curved mirrors of the invention desirably display a hemispherical reflectance (as measured using a reflectometer and integrating sphere over the wavelength range of 200 to 2600 nm) of at least 50% and a transmissivity not greater than about 4.0%. "Reflectance" herein is measured using a reflectometer utilizing a tungsten lamp at a filament temperature of substrates, the reflective coatings that are thus produced have not been uniform. The manufacturing process itself is tedious and time-consuming inasmuch as it requires multiple small glass substrates to be laid by hand upon a carrier that passes through a magnetron sputtering apparatus and requires each of the resulting individual mirror pieces to be removed by hand from the carrier sheet once the sputtering operation is complete.
To avoid these problems, it would be desirable to first sputter deposit a reflective coating on a flat glass sheet or other substrate to form a mirror, and then bend and cut the mirror as desired. However, a problem arises when flat glass sheets are coated with the usual reflecting layer using chromium, for example, as the reflective metal, and then are heat-bent. Once the coated sheets are heated to a temperature sufficient to enable permanent deformation that is, plastic flow--of the substrate to occur (approximately 1110-1130.degree. F. for glass), and the glass is bent, the coatings tend to develop defects which may be referred to as pits. The pits appear as visually detectable small, circular defects having little reflectance. The reason for the development of pitting is not fully understood, but is believed to be a function of stresses developed during the bending operation in one or more of the reflective sputter deposited films forming the reflective layer.